1. Field of the Invention
The present invention relates in general to mounting ophthalmic lens in a frame.
More particularly, the invention relates to a centering device adapted to detect automatically the position of one or more marks on an ophthalmic lens, and in particular the position of the mark(s) serving to identify a center and/or axis marking of an ophthalmic lens to be identified.
The invention also relates to methods of automatically detecting the positions of such marks and in particular a center and/or axis marking for an ophthalmic lens, and to two methods of manually centering an ophthalmic lens using such a centering and blocking device.
2. Description of the Related Art
Centering and blocking devices are appliances in widespread use in the field of optics. They are used in the process of fabricating a pair of spectacles, immediately prior to machining the ophthalmic lenses in order to fit them to the shape of the spectacles frame that has been selected.
Usually, a “raw” ophthalmic lens is substantially circular in shape, being of diameter that is sufficient to enable it to be mounted properly in the rim of the selected frame.
A centering and blocking device is then used to fix a handling peg on the ophthalmic lens in question, known as a “block”.
In a subsequent fabrication step, the handling peg is used for rotating the ophthalmic lens in order to machine it.
The handling peg is put into place on the front face of the ophthalmic lens at a point that is determined by calculation as a function in particular of the position of the “optical center” (in the broad sense of the term) or more generally the center point of the lens, the shape of the selected frame, and certain characteristics of the wearer, in particular the pupillary distance or half-distance and the height of the frame (the height of the pupils relative to the bottom portions of the rims of the frame).
Proper positioning or centering of the lens in the frame involves several components:                It is necessary to position the lens center point that faces the pupil of the eye (optical center for a single-vision lens, front-central point, center marking for a marked progressive lens, point determined from microetching of any progressive lens, or determined from the segment or marked point for a bi- or trifocal lens). Given the right (or left) shape of the lens, as read previously from a reader, and given the value of the half-spacings and the heights of the right (and left) eyes, it is possible to determine where the pupil of the right (or left) eye is to be found in this lens shape. Any offset between the center point of the lens and the pupil of the eye leads to prismatic effects that are undesirable for the wearer, and that are thus inconvenient. These effects are particularly troublesome when the lens is a high-power lens.        It is necessary to orient the axis of the lens in compliance with the prescription or the typology of the lens. For a cylindrical single-vision lens, the axis of the cylinder on the mounted lens must correspond with the prescribed axis. Any offset between the axes leads at least to poor correction (residual astigmatism) and that defect can be particularly troublesome when the cylinder is strong. With a progressive lens, the axis of the lens (as marked or as defined by microetching) must be horizontal in order to ensure that a lens of that type is properly mounted. For a bi- or trifocal lens, the segment must be horizontal not only for physiological reasons (good or bad correction), but also for reasons of appearance.        It is necessary to verify that the lens is large enough to occupy the frame.        It is necessary to verify that near correction (for a bi- or trifocal lens or for a progressive lens) is properly located in the frame. This can readily be understood from a physiological point of view (the wearer must be able to “access” the near vision zone of the lens in order to be properly corrected). In addition, for bi- or trifocal lenses, the segment must not be truncated, for reasons of appearance as well.        It is necessary to verify that far vision is well located in the frame, for the same physiological reasons. Complying with these various positioning components requires accurate measurement and viewing of the position of the center point, of the axis of the lens, of the outline of the lens, and of the positions of the reference points for near vision and for far vision (with a progressive lens).        
A center and/or clamping device is generally adapted to determine the position of an optical center for a single-vision lens, the position of one of the optical centers or any remarkable point known as a center point for a bifocal or trifocal lens, and is also adapted to determine some of the reference marks that the manufacturer usually causes to appear on the surface of progressive lenses. When the centering device is also a clamping device in the sense that it possesses means for manually or automatically placing a centering peg on the lens marking the detected reference point of the lens, the device is also adapted to determine by calculation a point on the surface of the lens that defines the location where the handling peg is to be placed.
Whether in automatic mode or in manual mode, most presently-known centering and blocking devices detect the position of the optical center or of the center markings and/or the axis markings of an ophthalmic lens by illuminating said lens with a light beam (generally a collimated beam) and by using a translucent projection screen to sense the light beam that has passed through the lens. A camera placed behind the screen acquires the projected image, and then displays it on a display peripheral such as a cathode ray tube (CRT) screen or a liquid crystal display (LCD) screen. The image of the lens as generated in this way is superposed on the shape of the frame so that the operator can achieve centering in all of its components, automatically or manually. In automatic mode, electronic processor means identify the shadows of center and/or axis markings in order to define a frame of reference for the frame and mark said frame of reference or deposit on the frame a reference or centering peg or block in the desired configuration for identifying a reference from for the lens on the basis of which the lens will be cut out to have the desired outline.
Such devices lead to errors in detecting the real positions of marks on the lens, and in particular the position of the optical center or of the center and/or axis markings of the ophthalmic lens (typically the mounting cross, the marking points obtained by centering on a frontofocometer, the horizontal lines, the microengraving, the outline of the segment of a non-progressive bifocal lens). The same applies to the circles, known as fronto-measurement circles, locating reference points for near vision and far vision. This error results from prismatic deflections of the shadows of the markings induced by the lens itself which deflections depend on the spherical, cylindrical, and prismatic optical powers of the ophthalmic lens in the zone of the marking in question. Because of these deflections, on going through the lens, the projected images of the markings on the convex front face of the lens are deformed, thereby inducing error in the various centering components and running the risk of subsequent mounting being incorrect or even impossible.
For example, a lens with positive power generates on the projection screen an image of the marks on the lens that is contracted overall. Conversely, a lens with negative power generates an image that is magnified. If the ophthalmic lens for centering presents lateral prismatic power in the region of the marking in question, the shadow of the marking on the image will appear to be offset laterally relative to the real position of the marking on the front face of said lens in the direction and by an amount that correspond to the angle of the prism. Similarly, if the ophthalmic lens presents toroidal power, then centering and blocking devices can commit an error in detecting the axis marking if the axis formed by the markings and the main axis of the corresponding torus are not parallel or mutually perpendicular.
In contrast, the outline of the lens is never deformed since it is not subjected to any prismatic effect. As a result there is a relative offset in the projection of said outline compared with the offset projections of the marks on the lens.
These errors are particularly important when the prismatic effects are strong and the projection screen is far away from the lens.
In an attempt to remedy that problem of error in detecting marks on a lens, proposals have already been made to minimize the offset errors that are produced by moving the projection screen closer to the lens. However under such circumstances such closeness can degrade the intrinsic position of the device and the compromises that have been envisaged remain unsatisfactory.
Another solution has been put forward in document EP 0 409 760 which relates to a centering and blocking device in which, firstly the optical path of the light beam used for detecting a position of the optical center or of the center markings of the lens is reversed, i.e. the ophthalmic lens is illuminated from behind (given that the center markings and/or axis markings are provided on the front face of the lens), and the light beam transmitted through the lens is picked up from beside its front face, and secondly the translucent projection screen for picking up the transmitted light flux in front of the acquisition means is disposed as close as possible to the front face of the lens for centering so as to limit the lengths of the paths followed by light beams that have been deflected prior to being focused onto the acquisition means.
Nevertheless, that requires the translucent projection screen to be movably mounted on the structure of the device in order to be retracted so as to allow the handling peg to be placed on the determined location of the front face of the ophthalmic lens. Such complex mounting of the projection screen on the structure of the device increases the size of the device, the cost of manufacturing it, and above all does not enable measurements to be obtained with long-lasting precision.